The damaging effect of the ultraviolet part of solar radiation on the skin is generally known. While rays having a wavelength of less than 290 nm (the so-called UVC range) are absorbed by the ozone layer in the earth's atmosphere, rays in the range between 290 nm and 320 nm, the so-called UVB range, cause erythema, simple sunburn or even burns of greater or lesser severity.
The narrower range around 308 nm is stated as the erythema activity maximum of sunlight.
Numerous compounds are known for protection against UVB radiation, these usually being derivatives of 3-benzylidenecamphor, of 4-aminobenzoic acid, of cinnamic acid, of salicylic acid, of benzophenone and also of 2-phenylbenzimidazole.
For the range between about 320 nm and about 400 nm, the so-called UVA range, it is also important to have available filter substances, since the rays thereof can also cause damage. Thus, it has been proved that UVA radiation leads to damage to the elastic and collagenic fibers of connective tissue, which makes the skin age prematurely, and that it is to be regarded as a cause of numerous phototoxic and photoallergic reactions. The damaging influence of UVB radiation can be intensified by UVA radiation.
However, UV radiation can also lead to photochemical reactions, the photochemical reaction products then intervening in skin metabolism.
Such photochemical reaction products are chiefly free-radical compounds, for example hydroxyl radicals. Undefined free-radical photo-products which are formed in the skin itself can also show uncontrolled secondary reactions because of their high reactivity. However, singlet oxygen, a non-radical excited state of the oxygen molecule, may occur under UV irradiation, as can short-lived epoxides and many others. Singlet oxygen, for example, is distinguished from the triplet oxygen normally present (free-radical ground state) by an increased reactivity. Nevertheless, excited, reactive (free-radical) triplet states of the oxygen molecule also exist.
UV radiation is furthermore counted as ionizing radiation. There is therefore the risk of ionic species also being formed during UV exposure, which then in turn are capable of intervening oxidatively in biochemical processes.
One advantageous UVB filter is 4,4',4"-(1,3,5-triazine-2,4,6-triyltriimino)tris-benzoic acid tris(2-ethylhexyl ester), synonym: 2,4,6-trisanilino-(p-carbo-2'-ethyl-1'-hexyloxy)!-1,3,5-triazine. ##STR1##
This UVB filter substance is marketed by BASF Aktiengesellschaft under the tradename UVINUL.RTM. T 150, and is distinguished by good UV absorption properties.
The main disadvantage of this UVB filter is the poor solubility in lipids. Known solvents for this UVB filter can dissolve not more than about 15% by weight of this filter, corresponding to about 1-1.5% by weight of dissolved, and therefore active, UV filter substance.
Other sparingly soluble UV filter substances are also known, for example 2-phenylbenzimidazole-5-sulphonic acid and its salts, in particular the sodium, potassium and TEA salt, for example obtainable under the name Eusolex.RTM. 232 from Merck AG, which is distinguished by the following structural formula: ##STR2##
Even if a certain UV protection can be achieved in principle with a given limited solubility (and therefore according to conventional standards: difficulty of incorporation into a cosmetic or dermatological formulation), another problem may occur, that of recrystallization. Precisely in the case of substances of poor solubility, this occurs comparatively rapidly, whether caused by variations in temperature or by other influences. Uncontrolled recrystallization of a substantial constituent of a formulation, such as a UV filter, however, has extremely adverse effects on the properties of the given formulation and, last but not least, on the light protection sought.
It is furthermore known that multiple emulsions--inter alia--can be distinguished by a particularly fine emulsion texture. This property renders them outstandingly suitable as a basis both for cosmetic and for medicinal topical formulations.
In simple emulsions, one phase comprises finely disperse droplets of the second phase enclosed by an emulsifier shell (water droplets in W/O emulsions or lipid vesicles in O/W emulsions). In a multiple emulsion (of the second degree), on the other hand, more finely disperse droplets of the first phase are emulsified in such droplets. In turn, even more finely disperse droplets can also be present in these droplets (multiple emulsion of the third degree) and so on.
Thus, as W/O or O/W emulsions (water-in-oil or oil-in-water) are referred to in the case of simple emulsions, in the case of multiple emulsions there are W/O/W, O/W/O, O/W/O/W, W/O/W/O emulsions and so on.
Multiple emulsions in which the particular internal and external aqueous phases or internal and external oily phases are of a different type (that is to say, for example, W/O/W' and O/W/O' emulsions) can be prepared by two-pot processes. Those emulsions in which the internal and external aqueous and oily phases are not of a different type are obtainable both by one-pot and by two-pot processes.
Multiple emulsions of the second degree are occasionally called "bimultiple systems", those of the third degree are occasionally called "trimultiple systems" and so on (W. Seifriz, Studies in Emulsions, J. Phys. Chem., 29 (1925) 738-749).
The expert is familiar per se with processes for the preparation of multiple emulsions. Thus, there are two-pot processes in which a simple emulsion (for example a W/O emulsion) is initially introduced into the preparation vessel and is converted into a multiple emulsion (for example a W/O/W emulsion) by addition of another phase (for example an aqueous phase) with a corresponding emulsifier (for example an O/W emulsifier).
A second known process comprises converting emulsifier mixtures with an oily phase and an aqueous phase into a multiple W/O/W emulsion in a one-pot process. The emulsifiers are dissolved in the oily phase and the solution is combined with the aqueous phase. A prerequisite for such a process is that the HLB values (HLB=hydrophilic-lipophilic balance) of the individual emulsifiers employed differ significantly from one another.
The definition of the ELB value for polyol fatty acid esters is given by the formula I EQU HLB=20.times.(1-H/A)
For a group of emulsifiers in which the hydrophilic content comprises only ethylene oxide units, formula II applies EQU HLB=E/5
where
H=hydrolysis number of ester, PA1 A=acid number of the acid recovered PA1 E=weight content of ethylene oxide (in %) in the total molecule.
Emulsifiers with HLB values of 6-8 are in general W/O emulsifiers, and those with HLB values of 8-18 are in general O/W emulsifiers. Literature: "Kosmetik-Entwicklung, Herstellung und Anwendung kosmetischer Mittel" Cosmetics-Development, Preparation and Use of Cosmetic Compositions!; W. Umbach (editor), Georg Thieme Verlag 1988.
Hydrophilic emulsifiers (with high HLB values) are as a rule O/W emulsifiers. Accordingly, hydrophobic or lipophilic emulsifiers (with low HLB values) are as a rule W/O emulsifiers.
U.S. Pat. No. 4,931,210 describes a process for the preparation of W/O/W emulsions in which polyglycerol polyricinoleates are used as emulsifiers.
The droplet diameters of the usual "simple", that is to say non-multiple, emulsions are in the range from about 1 .mu.m to about 50 .mu.m. Without further coloring additives, such "macroemulsions" are milky white in color and opaque. Finer "macroemulsions", the droplet diameters of which are in the range from about 10.sup.-1 .mu.m to about 1 .mu.m, again without coloring additives, are bluish white in color and opaque. Such "macroemulsions" usually have a high viscosity.
A clear and transparent appearance is reserved for micellar and molecular solutions with particle diameters of less than about 10.sup.-2 .mu.m, but these are no longer to be interpreted as true emulsions.
The droplet diameter of microemulsions, on the other hand, is in the range from about 10.sup.-2 .mu.m to about 10 .sup.-1 .mu.m. Microemulsions are translucent and usually of low viscosity. The viscosity of many microemulsions of the O/W type is comparable to that of water.
An advantage of microemulsions is that active compounds can be present in the disperse phase in a considerably more finely disperse form than in the disperse phase of "macroemulsions". Another advantage is that, because of their low viscosity, they can be sprayed. If microemulsions are used as cosmetics, corresponding products are distinguished by a high cosmetic elegance.
It is known that hydrophilic emulsifiers change their solubility properties from water-soluble to fat-soluble with increasing temperature. The temperature range in which the emulsifiers have changed their solubility is called the phase inversion temperature range (PIT).
S. Matsumoto (Journal of Colloid and Interface Science, Volume 94, No. 2, 1983) reports that the development of a W/O/W emulsion precedes a phase inversion of concentrated W/O emulsions stabilized by Span 80, a pronounced W/O emulsifier. Matsumoto starts here from an extremely non-polar oil, that is to say liquid paraffin. Moreover, a certain amount of hydrophilic emulsifiers is said to be necessary for development of a W/O/W emulsion from a W/O emulsion.
T. J. Lin, H. Kurihara and E. Ohta (Journal of the Society of Cosmetic Chemists 26, pages 121-139, March 1975) show that in the case of non-polar oils, extremely unstable multiple emulsions can be present in the region of the PIT.